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Abstract:

An image processing apparatus includes an influence value setting unit
that sets an influence value on a pixel of interest in a local region of
an image, the influence value being a color difference in the local
region, and a color region processing unit that treats the pixel of
interest as a pixel in a region of a representative color when a color of
the pixel of interest is within a range from the representative color to
the threshold value, the threshold value being set such that the
threshold value increases as the influence value increases.

Claims:

1. An image processing apparatus comprising: an influence value setting
unit that sets an influence value on a pixel of interest in a local
region of an image, the influence value being a color difference in the
local region; and a color region processing unit that treats the pixel of
interest as a pixel in a region of a representative color when a color of
the pixel of interest is within a range from the representative color to
the threshold value, the threshold value being set such that the
threshold value increases as the influence value increases.

2. The image processing apparatus according to claim 1, further
comprising: a boundary extracting unit that extracts a boundary between
colors, wherein the color region processing unit processes the region of
the representative color so as not to be continuous across the boundary
extracted by the boundary extracting unit.

3. The image processing apparatus according to claim 1, further
comprising: a boundary extracting unit that extracts a boundary between
colors, wherein the influence value setting unit sets larger influence
value as the pixel of interest approaches the boundary extracted by the
boundary extracting unit.

4. The image processing apparatus according to claim 2, wherein the
boundary extracting unit combines extraction results of the boundaries
between two or more color components.

5. The image processing apparatus according to claim 2, wherein the
boundary extracting unit extracts the boundary between colors using the
influence value set by the influence value setting unit.

6. The image processing apparatus according to claim 1, further
comprising: a representative color update unit that updates the
representative color by calculating a weighted average value for the
region of each representative color into which each pixel of interest is
incorporated by the color region processing unit, according to the
influence value of the pixel of interest.

7. A non-transitory computer readable medium storing a program causing a
computer to execute a process for processing an image, the process
comprising: setting an influence value on a pixel of interest in a local
region of an image, the influence value being a color difference in the
local region; and treating the pixel of interest as a pixel in a region
of a representative color when a color of the pixel of interest is within
a range from the representative color to the threshold value, the
threshold value being set such that the threshold value increases as the
influence value increases.

8. The image processing apparatus according to claim 3, wherein the
boundary extracting unit combines extraction results of the boundaries
between two or more color components.

9. The image processing apparatus according to claim wherein the boundary
extracting unit extracts the boundary between colors using the influence
value set by the influence value setting unit.

10. A method for processing an image, comprising: setting an influence
value on a pixel of interest in a local region of an image, the influence
value being a color difference in the local region; and treating the
pixel of interest as a pixel in a region of a representative color when a
color of the pixel of interest is within a range from the representative
color to the threshold value, the threshold value being set such that the
threshold value increases as the influence value increases.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on and claims priority under 35 U.S.C.
119 from Japanese Patent Application No. 2010-162810 filed on Jul. 20,
2010.

BACKGROUND

[0002] 1. Technical Field

[0003] The present invention relates to an image processing apparatus, a
computer readable medium for processing an image and a method for
processing an image.

[0004] 2. Related Art

[0005] As image processing, for example, the following processes have been
performed: a process that limits the number of colors used in an image to
several representative colors and divides the image into the regions of
the representative colors; and a process that converts the colors used
into representative colors and generates a limited color image. In the
processes, colors other than the representative colors in the image are
incorporated into the representative colors.

SUMMARY

[0006] According to an aspect of the invention, there is provided an image
processing apparatus including:

[0007] an influence value setting unit that sets an influence value on a
pixel of interest in a local region of an image, the influence value
being a color difference in the local region; and

[0008] a color region processing unit that treats the pixel of interest as
a pixel in a region of a representative color when a color of the pixel
of interest is within a range from the representative color to the
threshold value, the threshold value being set such that the threshold
value increases as the influence value increases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Exemplary embodiments of the present invention will be described in
detail based on the following figures, wherein:

[0010] FIG. 1 is a diagram illustrating the structure of a first exemplary
embodiment of the invention;

[0011] FIGS. 2A to 2G are diagrams illustrating an example of the
operation of the first exemplary embodiment of the invention;

[0012]FIG. 3 is a diagram illustrating an example of an influence value;

[0013]FIG. 4 is a diagram illustrating the structure of a modification of
the first exemplary embodiment of the invention;

[0014]FIG. 5 is a diagram illustrating the structure of a second
exemplary embodiment of the invention;

[0015] FIGS. 6A to 6D are diagrams illustrating an example of the
operation of the second exemplary embodiment of the invention;

[0016]FIG. 7 is a diagram illustrating the structure of a first
modification of the second exemplary embodiment of the invention;

[0017] FIG. 8 is a diagram illustrating the structure of a second
modification of the second exemplary embodiment of the invention;

[0018] FIGS. 9A to 9E are diagrams illustrating an example of an influence
value according to the second modification of the second exemplary
embodiment of the invention;

[0019]FIG. 10 is a diagram illustrating another example of the influence
value according to the second modification of the second exemplary
embodiment of the invention; and

[0020]FIG. 11 is a diagram illustrating an example of a computer program
for implementing the functions according to each exemplary embodiment of
the invention, a storage medium that stores the computer program, and a
computer.

DETAILED DESCRIPTION

[0021] FIG. 1 is a diagram illustrating the structure of a first exemplary
embodiment of the invention. In FIG. 1, reference numeral 11 indicates an
influence value setting unit, and reference numeral 12 indicates a color
region processing unit. When an image to be processed is given, the
following units sequentially process pixels in the image as pixels of
interest. In addition, one or plural representative colors used in the
image to be processed are extracted in advance.

[0022] The influence value setting unit 11 sets a color difference in a
local region that includes the pixel of interest and has a predetermined
size as an influence value on the pixel of interest. As the influence on
the pixel of interest increases, the influence value increases. The
influence value may be, for example, the difference (absolute value)
between the average color of the local region and the color of the pixel
of interest or the differential value (absolute value) of a variation in
the color of the pixel of interest. The total sum of the color
differences between the pixels in the local region or the maximum value
of the color difference may be used as the color difference.
Alternatively, the total sum of the products of the color difference
between the pixels and the distance between the pixel of interest and the
pixel closest to the pixel of interest among the pixels, or the maximum
value thereof may be used. In addition, the influence value may be
calculated by the following function using a combination of the color
difference and the differential value:

Influence value=α×color difference+β×differential
value+δ

[0023] (where coefficients α, β, and δ are positive
numbers).

[0024] The influence value may be calculated by, for example, the sum of
products or the product in addition to the linear sum. Of course, the
influence value may be calculated by characteristics other than the
above, or a combination of the characteristics. The influence value may
be set with reference to at least one of the color components. Of course,
the influence value may be set with reference to two color components or
three color components.

[0025] The color region processing unit 12 sets a threshold value
according to the influence value set by the influence value setting unit
11. When the color of the pixel of interest is within the range from one
of predetermined representative colors to the threshold value, the pixel
of interest is regarded as a pixel in the region of the representative
color. The pixels in the image are classified as the pixels of interest
into some regions of the representative colors. In this way, the pixels
are divided into the regions of each representative color. The color of
the pixel in the divided region is converted into the representative
color of each region, thereby limiting the color of the image.

[0026] FIGS. 2A to 2G are diagrams illustrating an example of the
operation of the first exemplary embodiment of the invention. FIG. 2A is
a diagram illustrating a portion of an image, in which color differences
are represented by different oblique lines. In this example, for example,
an emphasis process is performed such that there are a
lower-color-density portion and a higher-color-density portion at the
boundaries between a central portion with high color density and
low-color-density portions disposed on both sides of the central portion.
FIG. 2B shows a variation in the color density of a row of pixels
represented by an arrow in FIG. 2A.

[0027] In this example, the emphasis process causes a color that was not
present before the emphasis process to appear as disturbance in the
vicinity of the boundary. In addition, when an image reading apparatus
reads an image, the boundary between different colors is blurred due to
an error in reading. Therefore, the color of the blurred portion is
different from the original color. In the case of an image compressed by
lossy block coding, such as JPEG, it is confirmed that a color which has
not been used appears at the boundary between colors due to block noise.
As such, when disturbance is superimposed on an image to be processed and
a color different from the colors used appears, in some cases, the image
is divided into color regions including the region of the color different
from the colors used during the division of the color regions. In
addition, during conversion into a limited color image, in some cases,
the portion on which disturbance is superimposed is converted into a
different color.

[0028] The influence value setting unit 11 sets the influence value on the
basis of a variation in the color of the image. FIG. 2C shows the
absolute value of a variation in color difference from the average value
of a local region and FIG. 2D shows a variation in the differential
value. The influence value is calculated from these values by, for
example, the above-mentioned function. In this exemplary embodiment, it
is assumed that the influence value shown in FIG. 2E is calculated. The
influence value increases as the pixel of interest approaches a portion
with a large color difference, such as the boundary between colors. The
term "influence value" is used for a characteristic value obtained from a
color difference to facilitate understanding of the tendency that the
influence value of the pixel of interest increases as the pixel of
interest approaches the boundary between the colors.

[0029] The color region processing unit 12 sets a threshold value
according to the influence value set by the influence value setting unit
11 and determines the region of a representative color on the basis of
the threshold value. For example, as the influence value shown in FIG. 2E
increases, the threshold value increases, and as the influence value
decreases, the threshold value decreases. For example, the influence
value may be used as the threshold value. The threshold value set in this
way increases as the pixel of interest approaches a portion in which
there is a color difference and decreases when there is no color
difference. FIG. 2F shows the threshold value applied to the variation in
color density shown in FIG. 2B as a double-headed arrow line. The color
of the range of the double-headed arrow line is determined as the region
of a representative color. Referring to FIG. 2F, since the threshold
value increases as the pixel of interest approaches a portion in which
there is a color difference, the pixel of interest is likely to be
included in the region of an adjacent pixel. This shows that, even when
there is a variation in color at the boundary between the colors, the
pixel of interest is less likely to be affected by the variation.

[0030]FIG. 2G shows the processed image. In the example shown in FIG. 2A,
a variation in color density occurs in a portion in which different
colors are adjacent to each other, but is integrated into the region of
each representative color.

[0031]FIG. 3 is a diagram illustrating another example of the influence
value. In the example of the influence value shown in FIG. 2E, the
influence value is calculated from the color difference from the average
value of the local region and the differential value of the image.
However, FIG. 3 shows an example of easily setting the influence value.
For example, FIG. 3 shows an example in which the influence value is set
to 1 when the color difference from the average value of the local region
is equal to or less than a predetermined value and the influence value is
set to 2 when the color difference is more than the predetermined value.
As such, even when the influence value is set in this way, the color of
the pixel of interest is incorporated into the region of each
representative color without being affected by a color variation in a
portion in which colors are adjacent to each other.

[0032] In FIGS. 2A to 2G and FIG. 3, a one-dimensional variation has been
described. However, the local region may be a two-dimensional region and
the influence value may be set in two dimensions.

[0033]FIG. 4 is a diagram illustrating the structure of a modification of
the first exemplary embodiment of the invention. In FIG. 4, reference
numeral 13 indicates a representative color update unit. In this
modification, when a limited color image is formed, the representative
color of each region is calculated again.

[0034] The representative color update unit 13 calculates a weighted
average value weighted to an influence value corresponding to the pixel
of interest for the region of each representative color after the color
region processing unit integrates each pixel of interest and updates the
representative color. The weight is reduced as the influence value
increases. As described above, in some cases, the color of the pixel with
a large influence value is affected by different color regions.
Therefore, the influence of the color of the pixel is not reflected in
the update of the representative color. When the color of the pixel in
the color region is replaced with the representative color updated by the
representative color update unit 13, a limited color image with a color
identical to the color of the image to be processed is obtained.

[0035]FIG. 5 is a diagram illustrating the structure of a second
exemplary embodiment of the invention. In FIG. 5, reference numeral 14
indicates a boundary extracting unit. In the second exemplary embodiment,
the boundary extracting unit 14 is provided in addition to the structure
according to the first exemplary embodiment.

[0036] The boundary extracting unit 14 extracts the boundary between
colors from an image to be processed. A known method may be used to
extract the boundary between colors. For example, at least one color
component may be binarized on the basis of a local average value and the
binary boundary (changing point) may be extracted as the boundary between
colors. Of course, the boundary between colors may be extracted from two
color components or three color components. In this case, the boundaries
between colors extracted from the color components may be combined with
each other.

[0037] In the second exemplary embodiment, the color region processing
unit 12 performs integration into the region of each representative color
considering the boundary between colors extracted by the boundary
extracting unit 14. For example, the pixels (color regions) which are
adjacent to each other with the boundary between colors interposed
therebetween are not incorporated such that the region of the
representative color is not continuous across the boundary between
colors. In this process, for example, the threshold value of the
representative color in which the pixels adjacent to each other with the
boundary between colors interposed therebetween are incorporated may be
set to a small value, regardless of the influence value. When the
boundary between colors is considered, the boundary between colors in the
image to be processed is reflected in the boundary during the division of
the region at that position or the limited color image.

[0038] When setting the influence value, the influence value setting unit
11 may divide the binary image generated by the boundary extracting unit
14 into one value and the other value, thereby setting the influence
value. For example, when the value of the binary image corresponding to
the pixel of interest is one value, for example, the color difference or
the differential value may be calculated from the value of the pixel
equal to the one value of the corresponding binary image among the pixels
in a local region.

[0039] FIGS. 6A to 6D are diagrams illustrating an example of the
operation of the second exemplary embodiment of the invention. FIG. 6A
shows a variation in the color density of a row of pixels represented by
the arrow in a portion of the image shown in FIG. 2A. There are a
lower-color-density portion and a higher-color-density portion at the
boundaries between a central portion with high color density and
low-color-density portions disposed on both sides of the central portion,
which has been described in FIGS. 2A and 2B.

[0040]FIG. 6B shows the result binarized by the boundary extracting unit
14 on the basis of a local average value. The binary value varies at the
boundary between colors. The boundary extracting unit 14 extracts the
boundaries between colors. The regions a, b, and c are partitioned by the
boundaries between colors.

[0041]FIG. 6C shows an example of the influence value set by the
influence value setting unit 11. This example is shown in FIG. 2E.

[0042] The color region processing unit 12 sets a threshold value
according to the influence value set by the influence value setting unit
11 and determines the region of a representative color on the basis of
the threshold value. In this case, the region of the representative color
is not continuous across the boundary between colors extracted by the
boundary extracting unit 14. For example, the region b in the central
portion is not continuous to the region a or the region c across the
boundary between colors. In addition, neither the region a nor the region
c is continuous to the region b. For example, as described in the first
exemplary embodiment, a threshold value corresponding to the influence
value is set and integration into the color region is performed. In this
way, integration into the color region is performed without an influence
on a variation in the color at the boundary between colors. However, in
some cases, the integration is performed across the boundary between
colors. The boundary extracting unit 14 extracts the boundary between
colors and the color regions are not continuous across the boundary
between colors. In this way, the color regions are separated from each
other at the boundary between colors in the image to be processed.

[0043] In FIG. 6D, a threshold value applied to the variation in color
density shown in FIG. 6A is represented by a double-headed arrow line.
When the color of each pixel of interest is in the range from the
representative color to the threshold value, region integration is
performed. As represented by "x" in FIG. 6D, region integration is not
performed across the boundary between colors. Therefore, the color
regions are separated from each other at the boundary between colors.

[0044] A one-dimensional variation has been described with reference to
FIGS. 6A to 6D. However, the boundary between colors may be extracted in
two dimensions and the influence value may be set in two dimensions.

[0045]FIG. 7 is a diagram illustrating the structure of a first
modification of the second exemplary embodiment of the invention. In this
modification, the boundary extracting unit 14 extracts the boundary
between colors using the influence value set by the influence value
setting unit 11. The influence value indicates an influence due to a
color difference. Therefore, the influence value also indicates a portion
in which there is a color difference and may also be used to extract the
boundary between colors.

[0046] For example, the boundary extracting unit 14 extracts the boundary
between colors in an image portion in which the influence value set by
the influence value setting unit 11 is greater than a predetermined
value. Alternatively, when the boundaries between colors extracted from
plural color components are combined with each other, the color component
having an influence value smaller than a predetermined value may not be
used for the combination. Here, the influence value is calculated for
each color component by the influence value setting unit 11.

[0047] FIG. 8 is a diagram illustrating the structure of a second
modification of the second exemplary embodiment of the invention. In this
modification, the influence value setting unit 11 uses the extraction
result of the boundary between colors by the boundary extracting unit 14
to set the influence value.

[0048] When setting the influence value, the influence value setting unit
11 may set the influence value indicating that, as the pixel of interest
approaches the boundary between colors extracted from the boundary
extracting unit 14, an influence on the pixel of interest increases. For
example, the distance from the boundary between colors may be used to
calculate the influence value. For example, the influence value may be
calculated by the following function using a combination of, for example,
the color difference and the differential value described in the first
exemplary embodiment:

Influence value=α×color difference+β×differential
value-γ×distance+δ

[0049] (where coefficients α, β, γ, and δ are
positive numbers).

[0050] Alternatively, the sum of the products of the color difference or
the differential value and the reciprocal of the distance may be
calculated. Of course, the influence value may be calculated by the sum
of products or the product in addition to the linear sum, and
characteristics other than the above may be used combinedly to calculate
the influence value. In addition, the influence value may be set with
reference to at least one of the color components. Of course, the
influence value may be set with reference to two color components or
three color components.

[0051] FIGS. 9A to 9E are diagrams illustrating an example of the
influence value according to the second modification of the second
exemplary embodiment of the invention. FIG. 9A shows the binarization
result of the local average value by the boundary extracting unit 14,
which is shown in FIG. 6B. The binary value is changed at the boundary
between colors. FIG. 9B shows the absolute value of a variation in color
difference from the average value of a local region and FIG. 9C shows a
variation in the differential value, which are shown in FIGS. 2C and 2D,
respectively.

[0052] The distance from the boundary between colors is used and operates
in a predetermined range. The distance from the boundary between colors
is reduced as the pixel of interest approaches the boundary between
colors, and the distance from the boundary between colors increases as
the pixel of interest is farther away from the boundary between colors.
As shown in FIG. 9D, the distance from the boundary between colors is
treated as a value that is changed in a predetermined range. The function
expression of the influence value includes the distance as a negative
term. The absolute value of γ×distance is reduced as the
pixel of interest approaches the boundary between colors in the
predetermined range. Therefore, the influence value increases as the
pixel of interest approaches the boundary between colors. An example of
the obtained influence value is shown in FIG. 9E.

[0053]FIG. 10 is a diagram illustrating another example of the influence
value according to the second modification of the second exemplary
embodiment of the invention. In this example, in a predetermined range of
the distance from the boundary between colors, as the distance from the
boundary between colors is reduced, the influence value increases. This
may be used when it is easy to set the influence value, as compared to
when the color difference or the differential value is used.

[0054] In the examples shown in FIGS. 9A to 9E and FIG. 10, for example,
the color difference, the differential value, and the distance may be
calculated in two dimensions and the influence value may be set in two
dimensions.

[0055] In the second exemplary embodiment, the representative color update
unit 13 according to the modification of the first exemplary embodiment
shown in FIG. 4 may be provided to recalculate the representative color
in each color region.

[0057] The program 21 may allow the computer to implement all or some of
the functions of each unit according to the above-described exemplary
embodiments of the invention and the modifications thereof. In this case,
for example, the program and data used by the program may be stored in a
computer-readable storage medium. The storage medium causes a change in
the state of energy, such as magnetism, light, or electricity, in the
reading unit 43 provided in the hardware resources of the computer
according to the description content of the program, and transmits the
description content of the program to the reading unit 43 in the format
of signals corresponding to the change in the state. Examples of the
storage medium include the magneto-optical disc 31, the optical disc 32
(including, for example, CD or DVD), the magnetic disk 33, and the memory
34 (including, for example, an IC card, a memory card, and a flash
memory). These storage media are not limited to a portable type.

[0058] The program 21 is stored in the storage medium and the storage
medium is inserted into, for example, the reading unit 43 or the
interface 45 of the computer 22. Then, the computer reads the program 21
and stores the read program in the internal memory 42 or the hard disk 44
(including, for example, a magnetic disk or a silicon disk). The CPU 41
executes the program 21 to implement all or some of the functions
according to each exemplary embodiment of the invention and the
modifications thereof. Alternatively, the program 21 may be transmitted
to the computer 22 through the communication line, the computer 22 may
receive the program 21 using the communication unit 46 and store the
program in the internal memory 42 or the hard disk 44, and the CPU 41 may
execute the program 21 to implement all or some of the functions.

[0059] Various kinds of devices may be connected to the computer 22
through the interface 45. For example, a display unit that displays
information or a receiving unit that receives information from the user
may be connected to the computer 22. In addition, for example, an image
reading apparatus may be connected to the computer 22 through the
interface 45 and an image read by the image reading apparatus or an image
subjected to image processing may be processed by the process according
to each exemplary embodiment of the invention and the modifications
thereof. The region division result or the color-limited image after the
process may be transmitted to another program. Alternatively, the region
division result or the color-limited image may be stored in the hard disk
44 or in a storage medium through the interface 45, or it may be
transmitted to the outside through the communication unit 46. An image
forming apparatus may be connected to the computer through the interface
45 and form the processed color-limited image.

[0060] Some or all of the functions may be formed by hardware.
Alternatively, all or some of the functions according to each exemplary
embodiment of the invention and the modifications thereof and other
structures may be implemented by programs. When the program is applied to
other purposes, the program may be integrated with programs for other
purposes.

[0061] The foregoing description of the exemplary embodiments of the
invention has been provided for the purpose of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Obviously, many modifications
and variations will be apparent to practitioners skilled in the art. The
exemplary embodiments were chosen and described in order to best
exemplify the principles of the invention and its practical applications,
thereby enabling others skilled in the art to understand the invention
for various embodiments and with the various modifications as are suited
to the particular use contemplated. It is intended that the scope of the
invention is defined by the following claims and their equivalents.

Patent applications by Atsushi Itoh, Kanagawa JP

Patent applications by FUJI XEROX CO., LTD.

Patent applications in class Drop-out color in image (i.e., color to be removed)

Patent applications in all subclasses Drop-out color in image (i.e., color to be removed)